Semiconductor and other similar industries, often use optical metrology equipment to provide non-contact evaluation of substrates during processing. With optical metrology, a sample under test may be illuminated with light at a single wavelength or multiple wavelengths, e.g., at an oblique angle. After interacting with the sample, the resulting light is detected and analyzed to determine a desired characteristic of the sample. For example, the measured light from the sample may be compared to predicted light from a model. Desired parameters of the sample are varied in the model until a good fit is achieved between the predicted light and the measured light, at which time the modeled parameters are determined to be the characteristic of the sample.
Samples of interest typically have one or more films stacked on a substrate. In the semiconductor industry, and other similar industries, thin films ranging from a fraction of a nanometer to a few micrometers in thickness are used. The measurement of thin films using optical metrology is well known and considered straight forward. Industries, such as the semiconductor industry, however, are significantly increasing the thickness of films due to vertical stacking, which poses new challenges for conventional optical metrology techniques. For example, internal reflections of light within thick films produce complications that are difficult to overcome using conventional modeling techniques.